Generally, in an internal combustion engine performing lean combustion such as a diesel engine, an operation region in which an air-fuel mixture with a high air-fuel ratio (in a lean atmosphere) is combusted accounts for a large percentage of an entire operation region. Therefore, an exhaust path of the engine of this type is provided with a nitrogen oxide (hereinafter referred to as NOx) occlusion agent (an NOx occlusion catalyst) for occluding (absorbing) NOx contained in exhaust gas to purify the exhaust gas.
When the amount of NOx occluded in such an NOx occlusion catalyst reaches saturation, it is necessary to reduce NOx and restore the NOx occlusion catalyst. To reduce NOx, NOx reduction treatment is performed by adding an NOx reducing agent (fuel such as light oil) to the upstream of the NOx occlusion catalyst provided in the exhaust path, thermally decomposing the fuel to generate hydrocarbon, and using the generated hydrocarbon as a reducing agent to accelerate reduction of NOx.
Further, the exhaust gas of a diesel engine contains particulates (hereinafter referred to as PM (Particulate Matter)), soot, an SOF (Soluble Organic Fraction), and the like mainly composed of carbon, which are the causes of air pollution. As a device to remove such PM and the like, there has been known an exhaust gas purification device having a particulate filter provided in an exhaust path of a diesel engine for trapping PM contained in exhaust gas passing through the exhaust path to reduce the amount of emission released to the atmosphere. As a particulate filter, a DPF (Diesel Particulate Filter) or a DPNR (Diesel Particulate-NOx Reduction system) catalyst is used, for example.
When PM is trapped with a particulate filter and the trapped PM is deposited thereon in large amount to clog the particulate filter, pressure loss in the exhaust gas passing through the particulate filter is increased and accordingly an engine exhaust backpressure is increased, causing a reduction in engine output and fuel efficiency. To solve such a problem, PM regeneration treatment is performed by adding fuel to the exhaust path (upstream of the particulate filter) to increase an exhaust gas temperature and thereby accelerate oxidation (combustion) of the PM on the particulate filter.
As described above, in the NOx reduction treatment and the PM regeneration treatment performed to suppress deterioration of an exhaust gas purifying function of a catalyst, the exhaust path is provided with a fuel addition valve to supply fuel (a reducing agent) into the exhaust path. However, since an ejection hole of the fuel addition valve is exposed in the exhaust path, substances such as soot and an SOF contained in the exhaust gas adhere to and are deposited on the ejection hole of the fuel addition valve. Then, the adhering and deposited substances are exposed to the exhaust gas with a high temperature, altered and solidified to become deposits, thus causing clogging of the ejection hole of the fuel addition valve. As a method of preventing such clogging of the fuel addition valve, there has been proposed a method of decreasing a temperature at a tip of the fuel addition valve by forcibly performing fuel addition (hereinafter referred to as “anti-clogging addition”) at a timing other than that of adding fuel during NOx reduction and PM regeneration (see for example Japanese Patent Laying-Open No. 2003-222019).
Anti-clogging addition control is performed by predetermining an addition interval using an engine revolution number and an addition amount as parameters, and controlling opening/closing of the fuel addition valve based on the predetermined addition interval. However, when an engine is operated in a highland region or the like, an intake air volume is smaller than that in a flatland region, causing an increase in an exhaust gas temperature. When an environmental change (a change in atmospheric pressure) causes an increase in the exhaust gas temperature as described above, fuel addition control based on a predetermined addition interval may not be able to maintain a temperature at a tip of the fuel addition valve at not more than a predetermined value (i.e., a temperature at which production of deposits can be suppressed), and an ejection hole of the fuel addition valve may be clogged.